How to set min chunk size with turbopack?

[gavrilikhin-d]

Summary

Our waterfall is a ton of small chunks that spend most time waiting for server.

How can we merge those chunks?

Additional information

Using "slow 4G" throttling

Example

No response

[MILLERMARRU]

There's no direct minChunkSize knob exposed, but the config that actually controls this heuristic is experimental.turbopackChunkingHeuristics.requestCost. It's the estimated cost (in uncompressed bytes) Turbopack assigns to each additional request when deciding whether to merge chunks together, default is 200 KB, max 1 MB.

// next.config.js
module.exports = {
  experimental: {
    turbopackChunkingHeuristics: {
      requestCost: 1000000, // bump toward the max to bias hard toward merging
    },
  },
}

Raising it tells the algorithm "each extra request is expensive," so it merges more aggressively into fewer, bigger chunks instead of leaving you with the long tail of small ones you're seeing in the waterfall. That's exactly the tradeoff you want under throttled conditions like your slow 4G test, fewer round trips beats smaller individual payloads once RTT dominates.

If it's specifically your homepage/entry route that's suffering, there are two more knobs in the same block worth knowing: priorityRoutes (an array of RegExp matching routes you want merged more eagerly) and priorityBoost (how much more eagerly, default 1.5x). That combination lets you bias merging harder just for first-load-critical routes without changing the global default for the rest of the app.

[Willis0826]

Not sure if I’m misunderstanding this, but in the current Next.js docs, turbopackChunking does expose a minChunkSize option. I managed to reduce the number of small chunks by increasing minChunkSize. (using next 16.3.3)

[icyJoseph]

Starting with 16.3, turbopackChunking options were added, by this PR #96398

• https://nextjs.org/docs/app/api-reference/config/next-config-js/turbopackChunking#size-thresholds

One of those options is indeed, minChunkSize:

minChunkSize (default 50000): Turbopack will avoid creating more than one chunk smaller than this size by merging small chunks into larger ones. Raising this number will produce fewer, larger chunks. Meanwhile, lowering it will produce more chunks that are smaller.